1. Field of the Invention
The present invention relates to a magneto-optical recording/reproducing method and apparatus capable of reproducing record marks by displacing domain walls of record marks in a reproducing layer by utilizing a temperature distribution gradient generated upon application of an optical beam to a magneto-optical recording medium having a multi-layer film structure, without changing record data in a record storage layer, and by detecting a change in a polarizing plane of a reflected optical beam.
2. Related Background Art
A magneto-optical recording medium of an erasable high density record type is known in which information is recorded by writing magnetic domains in a magnetic thin film with thermal energy supplied from a semiconductor laser and the information is read by utilizing the magneto-optical effect. Needs for increasing the record density of a magneto-optical recording medium to realize a high capacity have recently increased.
A linear record density of a magneto-optical recording medium or disk depends greatly on a laser wavelength and a numerical aperture NA of an objective lens of a reproducing optical system. Specifically, since a diameter of a beam waist is determined from a laser wavelength .lambda. and a numerical aperture NA of an objective lens of the reproducing optical system, a detection limit of a spatial frequency during reproduction of record magnetic domains is about 2 NA/.lambda.. In order to realize high density in a conventional magneto-optical disk, it is necessary to shorten the laser wavelength and increase the NA of an objective lens, respectively, of the reproducing optical system. There is a limit, however, in improving the laser wavelength and objective lens NA. For this reason, techniques of improving a record density have been developed by devising the structure of a recording medium and a read method.
For example, Japanese Patent Application Laid-open No. 06-290496 proposes a signal reproducing method and apparatus. With this method and apparatus, signals are recorded in a record storage layer of a multi-layer film constituted of a reproduction (domain wall displacement) layer and a magnetically coupled record storage layer. Domain walls of magnetic domains in the reproducing layer are displaced by utilizing a temperature gradient of a recording medium generated upon application of a heating optical beam, without changing record data in the record storage layer, the reproducing layer is magnetized so that almost all of the area of the reproducing optical beam spot has the same magnetization state, and a change in the polarized plane of a reflected reproducing optical beam is detected to reproduce recorded magnetic domains which are at the refraction limit or more of the optical system.
With this method, recorded magnetic domains having a period shorter than a diffraction limit of an optical system can be reproduced. Therefore, the record density and transfer speed of a magneto-optical recording medium can be considerably improved.
A recording/reproducing system using one beam in the signal reproducing method and apparatus disclosed in the above-cited publication will be described.
The structure of a photomagnetic recording/reproducing apparatus using a one beam recording/reproducing method will be described with reference to FIG. 1. In FIG. 1, reference numeral 1 represents a magneto-optical disk which has a magnetic lamination film 3 and a protection film 4 formed on a substrate 2 made of glass or plastics. The magnetic lamination film 3 has a record storage layer and a domain wall displacement layer. Recorded magnetic domains can be reproduced by displacing domain walls of recorded magnetic domains in the domain wall displacement layer to broaden the magnetization area in a reproducing optical beam spot by utilizing a temperature distribution gradient generated upon application of an optical beam, without changing the record data in the record storage layer, and by detecting a change in a polarizing plane of a reflected optical beam. The magneto-optical disk 1 is supported by a spindle motor with a magnet chuck or the like, and is rotatable about the rotary shaft of the spindle motor.
Reference numerals 5 to 13 represent components constituting an optical head for applying a laser beam to the magneto-optical disk 1 and obtaining information from a reflected optical beam. Reference numeral 6 represents a condensing lens, reference numeral 5 represents an actuator for driving the condensing lens, reference numeral 7 represents a semiconductor laser as a light beam source, reference numeral 8 represents a collimator lens for converting an optical beam into a parallel light beam, reference numeral 9 represents a beam splitter for separating an optical beam, reference numeral 10 represents a .lambda./2 plate, reference numeral 11 represents a polarizing beam splitter, reference numeral 13 represents a photosensor, reference numeral 12 represents a condensing lens for condensing an optical beam toward the photosensor, and reference numeral 14 represents a differential amplifier for differentially amplifying signals having different polarizing directions.
A laser beam emitted from the semiconductor laser 7 is applied to the substrate 2 of the magneto-optical disk 1 via the collimator lens 8, beam splitter 9 and condensing lens 6. In this case, the condensing lens 6 is displaced along the focussing direction and tracking direction under the control of the actuator 5 so that the laser beam is focussed upon the magnetic lamination film 3, and also tracks a guide groove formed on the magneto-optical disk 1. An optical path of a laser beam reflected at the magneto-optical disk 1 and passed through the condensing lens 6 is changed by the beam splitter 9 toward the polarizing beam splitter 11. The laser beam is then picked up by the photosensors 13 as optical beams having different polarization angles depending upon the polarity of magnetization in the magnetic lamination film 3, via the .lambda./2 plate 10, polarizing beam splitter 11 and condensing lens 12. The outputs of the photosensors 13 are differentially amplified by the differential amplifier 14 which outputs a magneto-optical signal.
A controller 16 controls an LD driver 15, a magnetic head driver 18 and the like by supplying a record power, a recording signal and the like in accordance with input information such as a revolution number of the magneto-optical disk 1, a track position, a record radius, a record sector, a record start timing, a reproduction timing and the like. The LD driver 15 drives the semiconductor laser 7 while controlling the record power and reproduction power.
Reference numeral 17 represents a magnetic head for applying a modulating magnetic field to an area of the magneto-optical disk 1 where the laser beam is applied during the recording operation, and for applying a DC magnetic field during the reproducing operation. The magnetic head 17 is disposed facing the condensing lens 6, with the magneto-optical disk 1 being interposed therebetween, and is displaced to a proper position during the recording/reproducing operation.
During the recording operation, the LD driver 15 drives the semiconductor laser 7 with a DC record power, and at the same time a magnetic head driver 18 drives the magnetic head 17 to generate a magnetic field having a polarity corresponding to the recording signal. For example, record magnetic domains are formed through a magnetic field modulation record method.
The magnetic head 17 and the magnetic head driver 18 for magnetic field modulation are used for applying a DC reproducing magnetic field during the reproducing operation.
As the laser beam is applied, the magnetic head 17 displaces in a radial direction of the magneto-optical disk 1 to sequentially apply a magnetic field to the magnetic lamination film 3 in the area where the laser beam is applied to thereby record and reproduce information.
The magneto-optical signals are differentially amplified by the differential amplifier 14 whose output is supplied to a binarization circuit, PLL circuit, modulator circuit and the like (not shown) to reproduce information.
The guide groove of the magnetic lamination film 3 was formed in advance by annealing a guide groove area of the record medium at a high temperature to decompose it and not to make the domain wall of each recorded magnetic domain become a closed loop magnetic domain. This process facilitates a displacement of the domain wall.
The reproducing operation will be described with reference to FIGS. 2A to 2C. FIG. 2A is a top view showing a magnetic domain pattern on a domain wall displacement layer, FIG. 2B is a schematic cross sectional view of each magnetic layer, and FIG. 2C shows a temperature distribution of a medium.
In FIGS. 2A and 2B, reference numeral 21 represents a land, reference numeral 22 represents an annealed groove, reference numeral 23 represents an optical beam, reference numeral 24 represents a constant temperature line Ts, reference numeral 25 represents a downward magnetic domain, reference numeral 26 represents an upward magnetic domain, reference numeral 27 represents a domain wall displacement layer, reference numeral 28 represents a switching layer, and reference numeral 29 represents a record storage layer. S indicates a displacement direction of an optical beam, P indicates a domain wall arrival point, A indicates a domain wall displacement from an upstream area of the optical beam, and B indicates a domain wall displacement from a downstream area of the optical beam.
As shown in FIG. 2A, the magneto-optical medium is heated with an optical beam 23 to a temperature at which domain walls in the domain wall displacement layer can displace. Under this temperature condition, the constant temperature line 24 in a region above a critical temperature Ts of the medium at which domain walls can start displacing, becomes as shown in FIG. 2A. As shown in FIG. 2B, the record state of the record storage layer does not change, and the switching layer maintains constant in the temperature range equal to or higher than the critical temperature Ts of the medium. In the domain wall displacement layer, the magnetization state changes at the highest temperature. Different magnetization states exist both in the upstream and downstream areas along the optical beam advance direction. The domain walls displace both in upstream and downstream directions of the optical beam.
As shown in FIG. 2C, the highest temperature in the raised temperature area exists in the area where the optical beam is applied. A reproduction signal is obtained by synthesizing a signal generated by a displacement of a domain wall toward the highest temperature to be caused by the temperature gradient in the upstream area of the optical beam and a signal generated by a displacement of a domain wall toward the highest temperature to be caused by the temperature gradient in the downstream area of the optical beam.
FIGS. 3A to 3I are diagrams illustrating the read operation by the apparatus shown in FIG. 1. FIG. 3A is a diagram showing the waveform of a recording signal, FIGS. 3B to 3F are top views of record mark trains, and FIGS. 3G to 3I are diagrams of the waveforms of reproduced signals.
In FIGS. 3B to 3F showing the record mark trains, a circle mark with perfect circularity indicates an optical beam radiation area, and an ellipsoid mark narrowing along the downstream direction indicates an area higher than the critical temperature. By reproducing the record mark trains shown in FIGS. 3B to 3F formed by the recording signal shown in FIG. 3A, a reproduction signal shown in FIG. 3G is obtained while the record magnetic domains change with the advance of the optical beam as shown in FIGS. 3B to 3F. As described earlier, the signal shown in FIG. 3G is a signal shown in FIG. 3H generated by a displacement of the domain wall from the upstream area along the optical beam advance direction, superposed upon a signal shown in FIG. 3I generated by a displacement of the domain wall from the downstream area along the optical beam advance direction.
A change in the reproduction signal generated by a displacement of the domain wall from the downstream area along the optical beam advance direction may disturb the reproduction signal generated by a displacement of the domain wall from the upstream area along the optical beam advance direction, and the former signal is not necessary.
As one means for preventing a displacement of a domain wall from the downstream area along the optical beam advance direction, it has been proposed to apply a reproducing magnetic field. As a proper reproducing magnetic field is applied, a re-transfer of the domain wall from the downstream area along the optical beam advance direction, or a displacement of a domain wall of a re-transferred magnetic domain, can be suppressed. Therefore, the final reproduction signal is only a signal generated by a displacement of the domain wall from the upstream area along the optical beam advance direction. In this case, the displacement speed of a domain wall is high and the same magnetization state in the optical beam spot area occupies a half or more of the beam spot area while the domain wall displaces. Therefore, the reproduction signal or magneto-optical signal becomes rectangular.
However, if this means for applying a reproducing magnetic field is applied to the above-described conventional magneto-optical recording/reproducing apparatus, the strength of a reproducing magnetic field to be applied to a portable medium changes with the characteristics of each magneto-optical recording/reproducing apparatus.
The strength of a reproducing magnetic field to be applied also changes with each magneto-optical recording/reproducing apparatus because of a difference in the shape of an optical beam and hence a difference in a temperature distribution on a recording/reproducing medium.
The strength of a reproducing magnetic field to be applied also changes with an environmental temperature which influences the temperature distribution of the medium.
Since there are such changes in the strength of a reproducing magnetic field, a reproducing magnetic field having an insufficient or excessive strength may be applied. As a result, a displacement of the domain wall from the downstream area along the optical beam advance direction cannot be suppressed, and this signal generated by a displacement of the domain wall from the downstream area along the optical beam advance direction is superposed upon the signal generated by a displacement of the domain wall from the upstream area along the optical beam advance direction.
Therefore, a margin of the amplitude of a reproduction signal reduces and an error rate of reproduced signals increases, so that the recorded signals cannot be reproduced correctly.